1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission, particularly the technique of delaying initiation of gear change when the output shaft torque of the motive power source is changing.
2. Description of the Background Art
An automatic transmission that automatically effects gear changes based on the accelerator pedal position, the vehicle speed, and the like is conventionally known. In such an automatic transmission, the running state of the vehicle changes by variation in torque transmitted to the wheels by gear change. Such change in the running state caused by gear change may be felt as a jolt by the passenger, adversely affecting the drivability. It is therefore not always appropriate to effect gear change at the timing of the driver operating the accelerator pedal or at the timing of change in the vehicle speed. The approach of delaying the timing of effecting gear change with respect to the timing of determining gear change has been proposed.
Japanese Patent Laying-Open No. 02-080850 discloses a speed change control apparatus of delaying the engagement of the input clutch to conduct gear change when determination is made of a power-on downshift. The speed change control apparatus for an automatic transmission directed to a vehicle disclosed in the aforementioned publication is based on a planetary gear type transmission device having the input element switched by a first input clutch and a second input clutch. The speed change control apparatus includes a power-on downshift determination unit determining whether a power-on downshift is under way, i.e. downshifting to a lower gear implemented by the engagement of the first and second input clutches with a one way clutch from a higher gear implemented by engagement between the second input clutch and one reaction force element formation brake, and a delay unit delaying the engagement of the first input clutch when determination is made of a power-on downshift by the power-on downshift determination unit. Even if the first input clutch is released during engine driving at the gear implemented by the engagement of the first and second input clutches with the one way clutch, gear shifting can be implemented by engagement of the one way clutch. Further, even if the one way clutch is released during engine braking, gear shifting can be implemented by engagement of the first input clutch.
In accordance with the speed change control apparatus disclosed in the aforementioned publication, engagement of the first input clutch is delayed during a power-on downshift from the gear implemented by engagement between the second input clutch and one reaction force element formation brake to the next gear change implemented by engagement of the first and second input clutches with the one way clutch. Accordingly, the event of the first input clutch being engaged prior to engagement of the one way clutch can be obviated. During a power-on downshift mode in which the driver requests acceleration, the action of the engine brake on the vehicle caused by the one way clutch in a released state and the first input clutch attaining an engaged state can be suppressed. Thus, a power-on downshift can be effected without any great gear change shock.
The speed change control apparatus disclosed in the aforementioned publication merely effects delay for a period of time set with respect to the timing of gear change determination for gear shifting, and does not take into consideration the change in engine torque that may occur after determination of gear change. If gear change is effected corresponding to just a delay of the set time, there may be the case where the gear is shifted irrespective of the change in engine torque after gear change determination has been made. If gear change is effected ignoring the transitional change in engine torque subsequent to determination of gear change, the torque transmitted to the wheel (the output shaft torque of the automatic transmission) will change to cause an abrupt change in the vehicle acceleration. A gear change shock may occur by the abrupt change in acceleration. When gear change is effect under the state where the engine torque varies transitionally and the torque applied to the automatic transmission is unstable, it will be difficult to adjust the engagement pressure of the friction engagement elements of the automatic transmission so as to follow the change in torque. A gear change shock may occur in the case where the engagement pressure becomes higher than required with respect to the input torque (the case where the engagement pressure cannot be reduced in accordance with reduction in the input torque). The drivability will be degraded in all such cases.